It is common in the electronic art to encapsulate various devices to protect them from the ambient atmosphere in which they will be used. The word "device", singular or plural, is intended to include any type of component, electronic or not. Non-limiting examples of devices of particular commercial interest are electronic components, transistors, integrated circuits, LED's, acousto-optical devices, sensors, transducers, and so forth.
Plastic is a common encapsulant material for electronic devices. The device is typically coated with the plastic encapsulant by molding or potting. Two well known molding techniques are injection molding and transfer molding. With these techniques, the device is placed in a cavity in a mold, the mold closed and the plastic encapsulant is forced under pressure from a reservoir through various channels into the mold cavity so as to surround the device. Potting generally includes dipping the object into a liquid plastic or pouring a liquid plastic around the device. A mold may or may not be used for potting.
A great variety of plastic materials are used for encapsulation. Such materials and associated molding and potting techniques are well known in the art. In many cases, the electronic device is mounted on a leadframe or header before being encapsulated. As used herein, the word "leadframe" is intended to include any type of device support structure. Non-limiting examples are metal, ceramic or plastic headers, bases or circuit boards, and metal leadframes and pin-grid arrays, and combinations thereof.
A long standing problem with plastic encapsulation is that encapsulation materials with good mechanical and electrical properties, often do not adhere to the electronic device or its associated leadframe as well as is desired. By altering the composition of the encapsulant, it is possible to raise or lower its relative adhesion to different kinds of surfaces, e.g., metal, ceramic, oxide, nitride and semiconductor, to name a few.
Low adhesion is undesirable because it allows minute crevices to form where the plastic encapsulant separates from the surface of the leadframe or device. These crevices form pathways whereby ambient gases and moisture can penetrate and thereby degrade device performance, stability and reliability. Despite many years of effort by numerous researchers, it has not been possible to find means and methods for plastic encapsulation that simultaneously give both excellent mechanical and electrical properties and high adhesion of the encapsulant to leadframe and device surfaces. Accordingly, a need continues to exist for improved means and methods for encapsulating objects, especially electronic devices, that impart better adhesion without adverse impact on the mechanical and electrical properties of the encapsulation as a whole or on the finished device.